High temperature pebble bed reactor and process for shut-down

Abstract

A high temperature pebble bed nuclear reactor having a medium power capacity of 300 to 500 MW e is equipped with two different shut-down arrangements comprising reflector rods used exclusively for scram. The total shut-down reactivity of the reflector rods is proportioned in order to prevent the excessive cooling of the reactor core folowing scram. In this manner, the reactor core is rendered subcritical (in the event of accidents, for example, or at the beginning of any operating state) yet capable of returning to criticality at a reduced level of temperature and power output following the removal of heat. The use of all of the reflector rods for scram is effected only in the event of reactivity accidents. For all other scram incidents, only a portion of the reflector rods are used.

What is claimed is: 
     
       1. A process for the shutdown of a high temperature pebble-bed nuclear reactor having a core of spherical fuel elements, a reflector surrounding the core comprising a top reflector, a side reflector and a bottom reflector, means for the removal of decay heat, means for shutdown of the reactor over an extended period of time comprising at least one first absorber rod removably displaceable into the core of the reactor, and means for the rapid shutdown of the reactor comprising a plurality of second absorber rods removably displaceable into the side reflector of the reactor, the shutdown reactivity of said second absorber rods being such that the reactor may be reduced to a subcritical state under any condition by insertion of same into said side reflector, the process for shutdown of the reactor for all non-reactivity accidents comprising displacing only that portion of said second absorber rods into said side reflector which is sufficient to reduce the reactor to a subcritical state, the shutdown reactivity of said second absorber rods being such that only a portion of the total plurality of said absorber rods is required to achieve a subcritical state, removing decay heat to reduce the temperature of the reactor to a predetermined lower level and subsequently causing said reactor to become critical at said predetermined lower level. 
     
     
       2. The process of claim 1 wherein said predetermined temperature level is approximately 500° C. 
     
     
       3. The process of claim 1 wherein said reactor possesses a power capacity of from 300 to 500 MW el . 
     
     
       4. The process of claim 1 wherein all of the reflector rods are inserted into the reflector in the event of a reactivity accident. 
     
     
       5. The process of claim 1 wherein about one third to one half of the reflector rods are inserted into the reflector for an operational shut-down or nonreactivity accident.